The spin rate of golf balls is the end result of many variables, one of which is the distribution of the density or specific gravity within the ball. Spin rate is an important characteristic of golf balls for both skilled and recreational golfers. High spin rate allows the more skilled players, such as PGA professionals and low handicapped players, to maximize control of the golf ball. A high spin rate golf ball is advantageous for an approach shot to the green. The ability to produce and control back spin to stop the ball on the green and side spin to draw or fade the ball substantially improves the player's control over the ball. Hence, the more skilled players generally prefer a golf ball that exhibits high spin rate.
On the other hand, recreational players who cannot intentionally control the spin of the ball generally do not prefer a high spin rate golf ball. For these players, slicing and hooking are the more immediate obstacles. When a club head strikes a ball, an unintentional side spin is often imparted to the ball, which sends the ball off its intended course. The side spin reduces the player's control over the ball, as well as the distance the ball will travel. A golf ball that spins less tends not to drift off-line erratically if the shot is not hit squarely off the club face. The low spin ball will not cure the hook or the slice, but will reduce the adverse effects of the side spin. Hence, recreational players prefer a golf ball that exhibits low spin rate.
Aerodynamic forces acting on a golf ball are typically resolved into orthogonal components of lift and drag. Lift is defined as the aerodynamic force component acting perpendicular to the flight path. It results from a difference in pressure that is created by a distortion in the air flow that results from the back spin of the ball. A boundary layer forms at the stagnation point of the ball, B, then grows and separates at points S1 and S2, as shown in FIG. 1. Due to the ball backspin, the top of the ball moves in the direction of the airflow, which retards the separation of the boundary layer. In contrast, the bottom of the ball moves against the direction of airflow, thus advancing the separation of the boundary layer at the bottom of the ball. Therefore, the position of separation of the boundary layer at the top of the ball, S1, is further back than the position of separation of the boundary layer at the bottom of the ball, S2. This asymmetrical separation creates an arch in the flow pattern, requiring the air over the top of the ball to move faster and, thus, have lower pressure than the air underneath the ball.
Drag is defined as the aerodynamic force component acting parallel to the ball flight direction. As the ball travels through the air, the air surrounding the ball has different velocities and, accordingly, different pressures. The air exerts maximum pressure at the stagnation point, B, on the front of the ball, as shown in FIG. 1. The air then flows over the sides of the ball and has increased velocity and reduced pressure. The air separates from the surface of the ball at points S1 and S2, leaving a large turbulent flow area with low pressure, i.e., the wake. The difference between the high pressure in front of the ball and the low pressure behind the ball reduces the ball speed and acts as the primary source of drag for a golf ball.
An average professional can generally drive a golf ball at a speed of approximately 235 feet per second (ft/s) or 160 miles per hour (mph). Most amateur golfers, however, have a “lower swing-speed,” i.e., slower club head speed at impact compared to a professional golfer, and are able to drive the ball at a speed of about 130 mph and a distance of less than about 200 to about 240 yards. When compared to a ball hit by a high swing-speed player, a similar ball that is hit by a low swing-speed player travels along a more ballistic trajectory than the trajectory typically achieved by tour caliber players.
For example, when a player strikes a ball, a portion of the energy from the club head is transferred to the ball as ball speed, and another portion of the energy is transferred to the ball as ball spin. Players with low swing-speed will have less energy available to transfer to both ball speed and ball spin. When club speed becomes very low, the resulting ball speed can be low enough that the effect of ball spin does not significantly increase lift (FL), which, in turn, generates a low ball speed (V) and low lift (FL). Thus, the advantages of a golf ball designed to have beneficial flight properties, such as high spin and high lift, are minimized when hit by a low swing-speed player.
Low weight golf balls have been made in an attempt to increase the lift to weight ratio of the golf ball, thereby increasing the effects of the lift on ball trajectory, and also to produce a greater initial velocity upon impact than a heavier ball. It is generally known that low weight golf balls slow down faster than normal weight golf balls due to drag, an effect that is magnified at higher speeds. As a result, these low weight balls have not been effectively designed to decrease the effect of drag. Several attempts have been made in the past to minimize drag, but these attempts have been focused only in combination with a player having a higher swing-speed.
The dimples on a golf ball are used to adjust drag and lift properties of a golf ball and, therefore, the majority of golf ball manufacturers research dimple patterns, shape, volume, and cross-section in order to improve overall flight distance of a golf ball. The dimples create a thin turbulent boundary layer around the ball. The turbulence energizes the boundary layer and aids in maintaining attachment to and around the ball to reduce the area of the wake. The pressure behind the ball is increased and the drag is substantially reduced.
A high degree of dimple coverage is beneficial to flight distance, but only if the dimples are of a reasonable size. Dimple coverage gained by filling spaces with tiny dimples is not very effective, since tiny dimples are not good turbulence generators. Most balls today still have many large spaces between dimples or have filled in these spaces with very small dimples that do not create enough turbulence at average golf ball velocities. Generally, as the lift of a dimple pattern increases, drag also increases. Conventional dimple designs tend to be aerodynamically optimized for higher swing speeds than low swing-speed players can achieve.
The construction of the golf ball may also play an important role in the optimization of the flight characteristics of a golf ball. Over the past decade, advances in core and cover chemistry and layer construction have led to golf balls with improved in-play characteristics, such as initial velocity, spin rate and feel. Golf balls are typically constructed of a single or multilayer core, solid or wound, that is tightly surrounded by a single or multilayer cover formed of polymeric materials, e.g., polyurethane, balata rubber, ionomers, or a combination thereof. Golf balls with a low modulus thermoset polyurethane cover, for example, have inherent high spin rates, high drag levels, and manufacturing difficulties.
While past research has been focused on either on the optimization of golf ball aerodynamic properties or golf ball construction to make slight improvements in flight characteristics, most advances have benefited high swing speed players. In addition, most long distance prior art golf balls possess low spin at high launch angles and low lift coefficients, while most short distance prior art golf balls possess high spin at low launch angles and high lift coefficients. Both types of golf balls typically have high drag coefficients.
There is minimal prior art disclosing preferred aerodynamic characteristics for golf balls. U.S. Pat. No. 5,935,023 discloses preferred lift and drag coefficients for a single speed with a functional dependence on spin ratio. U.S. Pat. Nos. 6,213,898 and 6,290,615 disclose golf ball dimple patterns that reduce high-speed drag and increase low speed lift. It has now been discovered, contrary to the disclosures of these patents, that reduced high-speed drag and increased low speed lift does not necessarily result in improved flight performance. For example, excessive high-speed lift or excessive low-speed drag may result in undesirable flight performance characteristics. The prior art is largely silent, however, as to the combination of several aerodynamic features that influence other portions of golf ball flight, such as moment of inertia and flight consistency, as well as enhanced aerodynamic lift and drag coefficients for balls of varying size and weight.
A need thus exists for optimization of golf ball flight characteristics for all types of golfer swing speed, ability, or technique. In particular, a need exists in the art for a golf ball having a unique combination of lift and drag coefficients and spin rates.